In the drilling of oil, gas, and other types of wells in congested regions where there are numerous other existing wells, it is often necessary to have precise control of the path of the well being drilled to avoid other wells and to achieve the desired trajectory of the well bore in underground space. This control has generally been achieved in the past by periodic surveys of the well bore path in space using either magnetic-based or gyroscope-based surveying tools and steering of the drill path during drilling based on steering tools that sense the earth's gravity and magnetic fields. Typical of the magnetic field sensing steering tools is the Electronic Yaw Equipment (EYE) provided by Scientific Drilling International that is based on U.S. Pat. No. 3,791,043 "Indicating Instruments", and U.K. Patent 1240830 "Improvements In Or Relating To Indicating Instruments". This steering tool provides two magnetic field sensors and two acceleration or tilt sensors, all of which have their respective sensitive axes normal to the borehole axis in use.
The density of wells on offshore platforms or in localized drilling areas ashore has increased to the point where there may be many wells in close proximity to each other. It is often desired to drill a new well within two to five feet of other existing wells. The new well must be guided as it is drilled to maintain separation from existing wells and to achieve the desired trajectory in space. The magnetic fields caused by iron-based magnetic materials in adjacent well casings, platform structures, and other drilling apparatus are sufficient to cause large and unacceptable errors in the output of magnetic-based survey and steering tools. To date, this problem has been approached by using gyroscope-based survey tools to frequently survey the borehole path and magnetic-based steering tools to steer the well direction during drilling for short segments between such surveys. Such surveys are known as "single shot" gyroscope surveys. The gyroscope-based survey tools that have conventionally been used for these surveys are of the free directional gyroscope type that are not sufficiently rugged to remain in the hole for steering during subsequent drilling. Therefore, they must be withdrawn from the borehole before drilling is begun on each segment. Thus with previous methods, the direction of drilling for each segment is uncontrolled and very frequent stops must be made for repeated "single shot" surveys and short segments of blind drilling.
In borehole surveying and steering, the quantities usually used to describe the geometry of the problem are the azimuthal direction of the borehole with respect to either true or magnetic North, the tilt or inclination of the borehole with respect to the earth's gravity vector, the azimuthal tool face angle of the drilling apparatus, and the high side tool face angle of the drilling apparatus. These latter two quantities are measurements of the direction of a reference vector perpendicular to the borehole direction through a reference slot on the drilling apparatus. This slot is often referred to as a "muleshoe", a name derived from the shape of the slot. The azimuthal tool face is defined as the direction, with respect to North (either magnetic or true), of the horizontal projection of the reference vector. The high side tool face is defined as the rotation angle about the borehole axis from a vertical plane containing the borehole axis to the reference vector.
New gyroscope-based survey tools using inertial angular rate sensors rather than the older free-gyroscope direction reference approach provide improvements in surveying accuracy, speed, and flexibility. U.S. Pat. No. 3,753,296, "Well Mapping Apparatus and Method", U.S. Pat. No. 4,199,869, "Mapping Apparatus Employing Two Input Axis Gyroscopic Means", and U.S. Pat. No. 4,706,388, "Borehole Initial Alignment and Change Determination", are typical of such survey tools. Also U.S. Pat. Nos. 4,468, 863 and 4,611,405, both entitled "High Speed Surveying", described methods of using such inertial angular rate sensing survey tools. Since these new survey tools use inertial angular rate sensors rather than free directional gyroscopes they can be built to withstand the severe vibration environment during drilling and thus be used to steer the drilling process by providing measurements of azimuth, inclination, azimuthal tool face, and high side angle to the drilling operator.